"Doctors say he's severely dehydrated and low on vitamins and minerals," he said.

Alvarenga, an El Salvadoran who had been living in Mexico, washed ashore on the Marshall Islands eight days ago. Earlier this week, his condition improved enough that he was released from a hospital.

Doctors said Alvarenga's limbs have started to swell, and they can't seem to keep him hydrated.

Castaway Jose Salvador Alvarenga 5 photos

Castaway Jose Salvador Alvarenga5 photos

Castaway Jose Salvador Alvarenga – Jose Salvador Alvarenga attends a news conference in Majuro, Marshall Islands, on Thursday, February 6. Alvarenga, who is from El Salvador, said he spent 13 months lost in the Pacific Ocean, floating from Mexico to the Marshall Islands, which is about halfway between Hawaii and Australia.

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Castaway Jose Salvador Alvarenga5 photos

Castaway Jose Salvador Alvarenga – Alvarenga is helped up the stairs as he walks to the news conference in Majuro. He returned to a hospital to be intravenously fed, said a top Mexican trade official.

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Castaway Jose Salvador Alvarenga5 photos

Castaway Jose Salvador Alvarenga – A photo obtained on February 6 shows Alvarenga's boat shortly after his January 30 arrival on Ebon Atoll in the Marshall Islands. Alvarenga says that in December 2012, he and another man set off on a fishing trip from the port of Paredon Viejo, Mexico. He said they were blown off course by winds and then got caught in a storm, losing the use of their engines.

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Castaway Jose Salvador Alvarenga5 photos

Castaway Jose Salvador Alvarenga – Alvarenga waves as he walks with the help of a nurse in Majuro on Monday, February 3. Alvarenga said his companion died of starvation because he refused to eat raw birds and turtles.

Parents of castaway speak out

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But he did manage to leave the hospital briefly Thursday to make a few comments to the media.

Bewildered by all the cameras, he thanked the government and people of the Marshall Islands for their care and friendship. After his minute-long remark, officials whisked him back to the hospital.

Plans for his repatriation to El Salvador on Friday have now been postponed due to his health.

Official: His story 'is probable'

Alvarenga's claims have garnered widespread skepticism about how he could survive for 13 months adrift on the Pacific. But from what officials can tell, they have no reason to doubt him.

"The investigations into Mr. Alvarenga's story so far have been substantiated," said Phillip Muller, the Marshall Islands' foreign affairs minister.

Clay-Mendoza said Alvarenga was an undocumented worker in Mexico. But "what he has said to us about his identity, so far, has been true."

"We've had contact with his family in El Salvador, and they have corroborated his story," the Mexican official said. "Now we are trying to contact his boss at the fishing co-op he belonged to in Mexico, but so far everything he's told us has jibed. The main question now is how long was he at sea?"

Clay-Mendoza said "it's probable" Alvarenga really did get lost at sea starting in December 2012. He said the boat was reported missing in 2012.

If Alvarenga's story proves true, the trip across the Pacific would have taken him across roughly 6,600 miles (10,800 kilometers) of open ocean before ending in the Marshall Islands, about halfway between Hawaii and Australia, in the northern Pacific.